Adding Elixir logger

RE - https://github.com/phoenixframework/phoenix/issues/235

@chrismccord I need to test this, as well as add the stacktrace to the logger.error, still but this is the basic start. Once https://github.com/elixir-lang/plug/pull/75 gets merged or something like it I can complete this.
Seems better to raise something like `{:error, :unfetched}`. It seems a bit weird to make it return a string.